Adjusting reach to lift movements to sudden visible changes in target's weight

Abstract

People can adjust their reach-to-grasp movements online to sudden changes in the spatial properties of a target. We investigated whether they can also do this when a non-spatial property, weight, suddenly changes. Guiding your movement by using visual cues about an object's weight depends heavily on experience and is expected to be processed by the (slow) ventral stream rather than the (fast) 'online control' dorsal stream. In the first experiment, participants reached out and lifted an object with an expected or an unexpected weight. As predicted, there was an effect of expected weight on the time between the end of the reaching phase and the object's lift-off. In the second experiment, the object sometimes visibly changed weight after the participants had started their movement. The lifting time did not depend on whether the object had changed weight. Thus, participants can make online adjustments to a visually indicated change in weight. These results are interpreted as being contrary to existing theories of online control.